The present invention is concerned with electronic switching circuits comprising electromechanical relays and particularly but not exclusively to switching circuits for isolating electrical loads in the event of a fault.
Electromechanical relays are a very mature technology. Despite being replaced by semiconductor devices in many applications, the basic relay still retains many advantages over modern switching systems providing an inherently low voltage drop and good electrical isolation.
The lifetime of the electrical contacts of relays is usually the limiting factor in determining their usefulness in modern circuits. This is especially true for direct current (DC) applications where contact erosion takes place. The rated useful life of the relay contacts is often only 1% of the mechanical life, especially where loads which are controlled are inductive e.g. motor, or have a high in-rush current e.g. tungsten lamps.
Relays are still widely used in automotive applications as the system voltage for automobiles is relatively low e.g. 12V. At higher voltages, the material from which the relay contacts are made behave differently and contact erosion through arcing during the opening and closing of the contacts becomes significant.
At higher voltages, for example 36-48V, the ability of a relay to break a high current resulting from an electronic switch failure becomes a problem as the voltage across the contacts, which is sufficient to start and sustain an arc, can fuse the contacts together, resulting in an uncontrolled and unreliable disconnection. This can be particularly problematic in applications wherein the safety of a user is compromised. For example, if the voltage source is a battery in a powered vehicle such as a wheelchair or golf cart. Accordingly, there is a requirement to reliably disconnect the supply voltage from a load under conditions of voltage and/or current failures.